IP Multimedia Core Network Subsystem (IMS), which is the IP-based network architecture proposed by the Third Generation Partnership Project (3GPP for short), constructs an open and flexible service environment, supports multimedia applications, and is able to provide users with enriched multimedia services.
In the IMS service system, the control layer and the service layer are separated. The control layer does not provide specific services, but only provides necessary triggering, routing, billing and other functions to the service layer.
The service trigger and control functions in the control layer are accomplished by a call session control function (CSCF for short). The call session control function is divided into an proxy call session control function (Proxy-CSCF, P-CSCF for short), an interrogating call session control function (Interrogating-CSCF, I-CSCF for short) and a serving call session control function (Serving-CSCF, S-CSCF for short), among which the S-CSCF has the main responsibility, and the I-CSCF is optional.
The service layer consists of a series of application servers (AS for short), and is able to provide specific business services. The ASs might be standalone entities or reside in the S-CSCF.
The control layer (S-CSCF) controls service triggering according to user subscription information, invokes services in the ASs, and achieves service functions. Both the ASs and the S-CSCF can be collectively referred to as server equipment (SE for short).
Application layer gateways (ALG, including signaling processing entities and media processing entities) also exist in the IMS network to achieve interaction of control signaling and media between different networks. A session media path passing a ALG is divided into two sections, one being a intra-network media path, that is, a media path between the ALG and a terminal of a session which is in the same network as the ALG, the other being a inter-network media path, that is, a media path between the ALG and another terminal of the session.
End-to-end devices in the session are called as user equipment (UE for short) and are responsible for interacting with users. Some UEs use various network access modes, such as by a packet switch (PS for short) domain access network of 3GPP, by a PS domain access network of other non-3GPP, or even by a circuit switch (CS for short) domain access network, etc.
If an enhanced mobile switch center (eMSC for short) is configured in the CS network and provides a SIP (Session Initial Protocol) interface to interact with the IMS network, then the interaction between the IMS network and the CS network can be achieved by the eMSC.
For a UE with a variety of access modes, if the UE can use only one access mode at a time, when it is executing a certain service, such as a call, in one access mode, when the UE moves to other places and needs to change its used access mode, the UE and the network can provide a certain mode such that the service which is being executed by the UE will not interrupted. Such capability is called as single terminal radio voice call continuity, single radio voice call continuity (SRVCC) for short.
FIG. 1 is a flow chart of an existing method for implementing single radio voice call continuity, in which an IMS session between a UE-1 and a UE-2 is established, session signaling passes through an ALG and SCC AS (service centralization and continuity application server) in order to establish an IMS media connection path. After the single radio voice call continuity occurs in the media connection path through the ALG and the UE-1, the UE-1 and the network enable the UE-1 to establish a media connection using the CS domain and maintain the original session continuity process.
According to a standard single radio voice call terminal switching process, an access network decides to cause the UE-1 to switch from the PS domain to the CS domain, an eMSC prepares CS resources for the UE-1, which switches to the CS domain according to a network notification, where the process of keeping a single radio voice call continuity session alive comprises the following steps.
In step 101, a PS network entity, which is a mobility management entity (MME) or a serving GPRS support node (SGSN), sends a request of switching of the UE-1 from the PS domain to the CS domain to the eMSC.
In step 102, after the eMSC receives the request of switching of the UE-1 from the PS domain to the CS domain, it sends a transfer request destining for a number or identifier of the ALG to the ALG. The transfer request might be an INVITE message or an IAM (initialized address message).
In step 103, the ALG receives the transfer request and updates a media connection between the UE-1 and the ALG, that is, establishes a CS media connection between the UE-1 and the ALG.
In step 104, the ALG sends a response message of the transfer request, such as a “200 OK” message or an ANM (Answer Message), to the eMSC. Up to now, the ALG establishes a new signaling connection 4 with the eMSC.
During the switching from the PS domain to the CS domain, a CS signaling connection is also established between the UE-1 and eMSC.
Until now, a CS media connection is established between the eMSC and the UE-1 through the standard single radio voice call switching process, and a new media connection 3 is established through the single radio voice call continuity process. The eMSC connects the CS media connection with the newly established media connection 3, and the ALG connects the newly established media connection 3 with the original IMS media connection 2, such that the UE-1 can continue the call with the UE-2.
The existing method for implementing single radio voice call continuity does not consider an IMS re-registration timer in the S-CSCF. After the UE-1 switches to the CS domain, since the network does not provide a PS connection for the UE-1 to implement the IMS re-registration, the S-CSCF will release a session signaling path (that is, the signaling connection 2 in the FIG. 2) between the original SCC AS and the UE-1 due to re-registration timeout, such that the ALG receives a session release message in the IMS signaling connection 2, eventually resulting in interruption of the session.